In a CDMA mobile communication system, available frequency resources are divided into several bands and each band is in turn divided into several frequency points. In the system, each carrier frequency corresponds to a particular band and frequency point. At the time when mobile communication had not been fully developed, because of a small number of users, usually a single band deployed by an operator could meet communication requirements. However, since the single band could just provide a limited capacity, as the number of users increases, the single band cannot fully meet requirements of the users, the operator must deploy more bands to accommodate more users.
In a multi-carrier mobile communication system, regardless of whether carrier frequencies belong to a same band, load balance between the carrier frequencies must be considered. If traffic is concentrated in one or more of the carrier frequencies, this will inevitably lead to heavier load on some of the carrier frequencies, which affects functionalities of the service or quality of the communication, and even damages devices, while idle carrier frequencies are not fully utilized, resulting in a waste of resources.
In a CDMA system, a base station issues a CDMA Channel List Message to terminals in an idle state. The message contains information of some or all frequency points in all carrier frequencies at a current band deployed by the system. Each terminal calculates its own standby frequency point by using a HASH algorithm based on its International Mobile Subscriber Identification Number (IMSI). The HASH algorithm can make the number of standby terminals on each frequency point almost as same as each other. Balancing among the numbers of standby terminals in the idle state can bring an service state balancing to a certain extent, and thus in the whole, the load on each carrier band among a same band can be balanced substantially.
However, this balancing method cannot be applied to a network in which a plurality of bands are deployed, because the CDMA channel list message does not carry band information. The terminals perform the HASH algorithm just in the band in which the CDMA channel list message is received and on the frequency points listed in the message. Therefore, there is no way to balance the loads among the bands.
Chinese patent application No. 200710077248.8, entitled “,    (Base Station, Method and System for Adjusting Band Loads in Multi-band Network)” proposes a method to solve the problem of load imbalance among the bands. In the patent, a global redirect message is used to adjust numbers of standby terminals in various bands, in order to implement inter-band load balance. However, this method also has the following disadvantages: first, just a balancing method for terminals in the idle state is provided, while balancing in other states is not mentioned, and thus it is a limited method; and second, the global redirect message depends merely on Access Overload Class of the terminal to determine the band for the standby terminal, and thus only a single adjusting approach is employed.
U.S. patent application No. US2007110021, entitled “Method and system for directing a call for a mobile station to a band class in a wireless communication network” provides a method of adjusting band loads by assigning a traffic channel across bands. When a terminal accesses the network, if there is a heavy load in the accessed band, the base station indicates the terminal to jump to another band with lighter load to establish a call, such that the loads in different bands can be adjusted. However, this method only provides an inter-band balancing method for terminals in an access state, but does not provide a balancing method in other states, and thus also has a great limitation.